Re: [ATM] Elliptical mirror

[Aplanatic@aol.com]

Hello Malcolm,

You must have an optical prescription for the Gregorian secondary, no?  The 
mirror's surface is a conic of revolution.  In our optical world the amount the 
surface differs from a sphere is called the conic deformation constant or the 
Schwartzschild Constant (SC).  For a sphere the deformation constant is zero, 
for a paraboloid it is -1.  For a prolate ellipsoid it is between -1 and 
zero, and for an oblate ellipsoid it is greater than zero.  This is the number 
that you must enter into Figure45 or FigureXP.

If the desired surface is a prolate ellipse then use the same techniques as 
you would to make a paraboloid, only stop short.  

If the surface is an oblate ellipsoid then I personally don't have a figuring 
strategy for you.  I'd experiment with strokes that tend to remove more glass 
from the 70% zone.  I'd probably try a non-uniform lap and then smooth back 
to the desired shape.

Dave Rowe

>  need some help with figuring an elliptical secondary for a Gregorian
>  telescope ( well two actually). There is plenty of  information available
>  about  the ellipse, but it all concerns how it can be avoided or eliminated
>  and little about how to create one.
>  
>  My first question is:  How do I set 'Fig XP' for an ellipse.  I presume I
>  must alter the desired deformation number. What is the magic number that I
>  alter it to?
>  
>  Second question:  Is there any special stroke that will produce an ellipse.
>  I have pressed  paper stars into the centre and outside edge of the lap. Is
>  that the correct thing to do?
>  
>  Any other tips or advice from an experienced person would be appreciated.
>  
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